Most new mods for FNV and TTW are gameplay and script based due to the continued progression on NVSE proper along the JIP and Lutana plugins and are incompatible with FO3. As far as asset creation that'll stay transferable, but really there have been more assets released for FO3 and FNV in the past couple months. The TTW community doesn't really seem to have many asset artists.
In terms of FO4, yes some assets will be usable. As an example, I have not seen a high quality retexture of mushrooms in FO3/FNV that retains the same look. FO4 however has a higher resolution version of the FO3 texture. Bethesda keeps the original assets used to create meshes and textures. To see for yourself:
FO3: \textures\landscape\plants\wastelandfungusstalk01.dds x256
FO4: \Textures\Landscape\Plants\wastelandfungusstalk01_d.dds x512
Additionally there are things like the high poly Chinese Rifle from FO3 that sits unused in the FO4 files.
The problem however, is that these are not drag and drop like FO3/FNV. They have to be converted back like you would if transferring assets between Oblivion and Skyrim. While you can do this on your own, this makes it difficult for most people to use as you cannot of course distribute FO4 assets.
EDIT: I should add that obtaining permission and porting original FO4 mod assets is fine, though not something I've looked at yet. For example, I obtained permission to port assets from SMIM from Skyrim over to FO3/FNV, particularly some of the food that shared meshes between FO3 and Skyrim, though have not released it on the nexus yet.
Edited by Adonis_VII, 28 December 2016 - 02:54 AM.